1. Field of the Invention
The present invention relates to golf balls. More particularly, the present invention relates to golf balls having a core and a cover, with dimples being formed on the cover.
2. Description of the Related Art
General golf balls have a core and a cover. There exist the cores composed of a single solid rubber layer, those composed of two or more solid rubber layers, those composed of a solid rubber layer and a synthetic resin layer, and the like.
Numerous dimples are formed on the surface of the cover. A role of the dimples involves causing turbulent flow separation through disrupting the air flow around the golf ball during the flight. By causing the turbulent flow separation, a separating point of air from the golf ball shifts backwards leading to the reduction of a drag coefficient. The turbulent flow separation promotes the differentia between the separating points at the upper and lower sides of the golf ball, which result from the backspin, thereby enhancing the lift force that acts upon the golf ball.
A variety of dimples to which improvement of the flight performance is intended have been proposed. For example, improvement in connection with the number, area, volume, the cross-sectional shape and the like of dimples has been attempted (see. U.S. Pat. No. 5,292,132).
For the formation of a cover, injection molding or compression molding has been employed. In either case of the molding, a molten resin composition flows in the space between a cavity face of a mold and a core. When the flow is insufficient, failures such as air residual, bare, off-center of the core and the like are caused. A variety of resin compositions to which improvement of the fluidity (i.e., formability) is intended have been proposed. For example, U.S. Pat. No. 6,624,221 discloses a resin composition for a cover including a thermoplastic elastomer and a wax blended. U.S. Pat. No. 5,872,185 discloses a resin composition including an ionomer resin and a thermoplastic styrene elastomer blended.
Control performances are also important for golf balls. The control performance correlates with a spin performance. Great back spin rate results in small run (a distance from the position where the golf ball dropped to a position where it stopped). For golf players, golf balls which are liable to be spun backwards are apt to be rendered to stop at a targeted position. High side spin rate results in possibility of curving of the golf ball. For golf players, golf balls which are liable to be side spun are apt to be rendered to intentionally curve. Senior golf players particularly place great importance on control performances when the golf ball is hit by a short iron.
As described above, the back spin results in generation of a lift force. Although the lift force is necessary for the flight performances of a golf ball, excess lift force may rather shorten the flight distance by contrast. Particularly, when the lift force that acts on a golf ball which is flying at a high speed following impact with a driver is too great, hopping of the golf ball is provided, leading to significant reduction of the flight distance. An object of the present invention is to provide a golf ball which is excellent in formability, a control performance and a flight performance.